Sandwich-structured BOPET films with high energy density and low loss for high temperature film capacitors

Film capacitors have widely been used in modern electrical/electronic equipment because of their high insulation property and high-power density. However, the traditional dielectric films suffered a high conduction loss under high temperature and electric field conditions, which leads to the depression of high temperature energy storage properties. Herein, sandwich-structured composite films using biaxially oriented polyethylene terephthalate (BOPET) as the outer layers were prepared to construct the interfacial barrier, which was beneficial for hindering the propagation of breakdown path. In addition, epoxy resin was applied as the adhesive inner layer to bind the outer layers up tightly, while the boron nitride nanosheets (BNNSs) in the inner layer further improved the breakdown strength of the film and inhibited the conduction loss, especially under high temperature and electric field conditions. The composite film with optimized BNNSs content had a high discharged energy density of 9.11 J/cm3 and a ultrahigh charge–discharge efficiency of 95% at 25 °C, while the values were 6.45 J/cm3 and 70% at 150 °C, respectively, of which both were much higher than those of pure BOPET. Thus, the sandwich-structure method points out a promising way of preparing dielectric films with excellent energy storage performances.